Allow drivers to alter the default driver name on rsdHalInit

Bug: 20894432

Permit partner drivers via Context::setDriverName to choose their
own mDriverName rather than the default OVERRIDE_RS_DRIVER string.

This can only be done once from within rsdHalInit otherwise it will
be ignored and the default behaviour will apply.

Change-Id: I49b81e60f7983a81f0e71fbef4b46f9bfc834849
2 files changed